Halftone color separations are used as copy originals for making offset and letterpress printing plates in reprographic processes. Before the printing plate is exposed, the color separation is proofed to determine whether the subsequent printing result will be a tonally correct reproduction of the original.
Such proofing processes use, for example, radiation-sensitive recording materials on which the image is produced by differences in the tackiness of the exposed and unexposed areas of the radiation-sensitive layer. Thus, for example, German Patent DE-C 12 10 321, U.S. Pat. No. 3,620,726, U.S. Pat. No. 3,582,327, U.S. Pat. No. 3,649,268, U.S. Pat. No. 4,356,253, and U.S. Pat. Nos. 4,948,704 and 4,243,741 disclose a reproduction process, wherein a tacky photopolymerizable material, comprising a support and a photopolymerizable layer containing at least one addition-polymerizable monomer and a photopolymerization initiator, is hardened by imagewise exposure. The exposed image areas lose their tackiness.
The latent image is made visible by the application of suitable toners that adhere only to the unexposed tacky areas and can be removed from the exposed hardened areas. This process yields a positive, optionally color, image of the original, similar in appearance to images prepared by the use of printing inks.
Negative images of the original are obtained by the use of photosensitive materials described, for example, in U.S. Pat. Nos. 4,243,741, 4,346,162, and 4,604,340. The photosensitive component of these recording materials is either a dihydropyridine compound or a system of a dihydropyridine compound and a hexaaryl bisimidazole compound.
The toners, comprising predominantly finely divided powders, can be applied by being dusted on the imagewise exposed surface, as described, for example, in U.S. Pat. Nos. 4,215,193 and 3,909,282. In another embodiment, the toners are loosely bonded on a special carrier and are transferred by bringing the carrier into contact with the imagewise exposed layer. Such pigmented carriers are described in U.S. Pat. Nos. 4,806,451 and 5,090,774.
The radiation-sensitive recording materials usually comprise a support, a radiation-sensitive layer, and a cover sheet. This cover sheet is stripped off, and the material is laminated on an image carrier before imagewise exposure. The support for the recording material can be stripped off before or after exposure depending on the material.
Standard line screens having a resolution of 60 lines per centimeter are currently used in reprography. Line screens having a resolution of 80 lines per centimeter are used for high quality printing. However, higher resolution screens are needed for especially high quality printing. Examples of such high resolution screens have 100 to 120 lines per centimeter or are frequency modulated screens which have very small equally sized spots that are randomly placed.
The use of high resolution screens, particularly frequency modulated screens, in reprography has been highly limited until now by a lack of suitable color proofing processes. The color proofing processes now on the market are not adequate to make monochromatic or polychromatic proofs corresponding to subsequent printing results. In particular, the very small dots in frequency modulated screens, having a dot diameter less than 25 .mu.m and the 2% dots in line screens having a resolution above 80 lines per centimeter are not satisfactorily captured by current proofing processes. This yields color proofs that the expert designates as "uneven", "spotty", "hazy", or full of "mottle". Such color proofs are not suitable for testing printing masters.
Therefore, color proofing processes are required for modern planographic printing by the graphic arts industry for halftone color separations made with high resolution screens to enable proofing the preparation of printing plates.
Image carriers for color proofing processes are known to the expert. However, no image carriers for color proofing processes have been proposed until now to enable a true-to-the-original reproduction from high resolution halftone masters with a dot diameter of less than 25 .mu.m.
The problem involved in this invention is to propose a color proofing process for preparing high resolution mono-chromatic and polychromatic proofs and to propose a process and image carrier for their preparation.
This problem is solved by monochromatic and polychromatic proofs according to the present invention, by image carriers for the preparation of such color proofs and by processes for the preparation of monochromatic and polychromatic proofs.